Multimedia communication and its associated system hardware are known. One or more examples are described in the recommended standards published by the International Telecommunication Union (ITU). lTU is an agency of the United Nations specialized in the field of telecommunications. The ITU is responsible for studying technical, operating and tariff questions; and it issues recommendations about them with the view to standardize telecommunications on a world wide basis. One such recommended standard is found in the series H: Audiovisual and Multimedia Systems, H.323: Visual Telephone Systems and Equipment for Local Area Networks, and H.245: Control Protocol for Multimedia Communication. A copy of the recommended standards may be obtained by contacting the United Nations, New York, N.Y., USA.
Since such recommendations are for the world wide use, the standard are incorporated with many provisions for different equipments and systems around the world. To provide multimedia communication, the terminals connected to a shared network medium, such as a local area network, are required to have a minimum hardware resource. All H.323 terminals shall have an audio codec, and be capable of encoding and decoding speech according to predefined recommendations. A video codec is optional. If video codec is used in the terminal device, the video encoding and decoding are according to predefined recommendations. A system control unit provides signaling for proper operation of a H.323 terminal. The system control unit operates according to H.245 standard. To carry multimedia communication between two end points, such as two terminals, a control channel and a logical channel according to H.245 are established between the terminals.
A terminal may receive more than one audio channel, particularly in a multipoint conference call involving the terminal. In a multipoint conference call, therefore, there is a need for performing an audio mixing function in order to present the composite audio signal to users of the multipoint conference call. The terminals indicate via H.245 signaling the information regarding the number of simultaneous audio streams capable of decoding and encoding.
In addition, there is a need for a multipoint controller which provides for the control of three or more terminals participating in a multipoint conference call. The multipoint controller also may connect two terminals in a point-to-point call which later may develop into a multipoint conference call. The multipoint controller provides capability negotiation with all terminals to achieve common levels of communications. In addition, it may control conference resources such as multicasting video. The multipoint controller does not perform mixing or switching of audio, video and data.
The multipoint controller may be a stand alone terminal connected to the shared network medium, or it may be a part of a terminal generating or receiving audio and video signals. If the multipoint controller is stand alone, it may additionally have a multipoint processor. Multipoint processor provides for a centralized processing of audio, video and data in a multipoint conference call. The multipoint controller and multipoint processor may be combined in a unit called multipoint control unit. The multipoint control unit communicates to other entities, such as other terminals, according to H.323 standard.
There are normally a gateway and a gatekeeper connected to the shared network medium. The gateway connected to the shared network medium provides a real time two-way communications between terminals complying with H.323 standard and terminals connected to other networks. The gatekeeper provides address translation and control access to the local area network for H.323 terminals, gateways, and multipoint control units. A call between two or more terminals may be a direct call between the two terminals or via the gatekeeper.
To have a multipoint conference call, the call should be established as a multipoint conference call, or switched to a multipoint conference call after the call has initially been established. If the initial call is between two terminals, the call may be switched to a multipoint conference call only if at least one of the terminals has a multipoint controller or if initially resources of a multipoint control unit was allocated in the call setup. If the call is initially setup between two terminals, normally, there is no need to allocate resources of a multipoint control unit, unless multipoint conference call feature was initially requested. If neither of the terminals has a multipoint controller, the call between two terminals can not be switched to a multipoint conference call because the multipoint controller resource, which is required for multipoint conference call, can not be added to the control signaling after the initial call set up due to lack of provisions made in H.323 standard.
There are many terminals built and used around the world by complying with H.323 standard, such terminals may not have internal multipoint control resources in order to save cost and complexity of the terminals. The multipoint control resource may be allocated via a multipoint control unit at an address location on the shared network medium. Therefore, for the terminals that do not have a multipoint control resource, and the call initially has not been setup with allocation of multipoint control resource, switching the call into a multipoint conference call is impossible, unless the initial call is terminated and a new call with multipoint control resource is established.
Therefore, there is a need to provide for a method that allows terminals without multipoint control resource while complying with H.323 standard to switch a call to a multipoint conference call without reestablishing a new call.